A light transmitting structure or optical waveguide (47) of the present
invention can provide both optical and electrical conductivity. The
optical waveguide (47) is capable of simultaneously transmitting multiple
optical and electrical signals. The optical waveguide (47) has several
cladding layers (40, 41, 42, 43, 44) which optically isolate several
different regions of waveguide core material (45, 46) through which the
multiple optical signals are transmitted. To provide a compact structure,
portions of the cladding layers (42, 43, 44) of the optical waveguide (47)
are used as electrical conductors while the waveguide core material (45,
46) is used as an electrical insulator for the electrical conductors.

a waveguide core material over the first polyimide layer and between at
least a portion of the at least two metal structures, the waveguide core
material having a first index of refraction and comprising an optically
conductive adhesive; and

a second polyimide layer over the at least two metal structures.

2. The polyimide optical waveguide according to claim 1, wherein each of
the at least two metal structures further comprise:

a first metallic seed layer over the first polyimide layer; and

a plated metal layer over the first metallic seed layer.

3. The polyimide optical waveguide according to claim 2, wherein the second
polyimide layer is over the waveguide core material and further comprising
a second metallic seed layer between the waveguide core material and the
second polyimide layer, wherein the waveguide core material is over the
first metallic seed layer and wherein the first metallic seed layer is
between the waveguide core material and the first polyimide layer.

4. The polyimide optical waveguide according to claim 1, wherein the at
least two metal structures are electrically conducting and wherein the
waveguide core material is electrically insulating such that the polyimide
optical waveguide is both optically and electrically conducting.

5. The polyimide optical waveguide according to claim 4, wherein the at
least two metal structure are electrically isolated from each other by the
waveguide core material and by the first and second polyimide layers.

6. The polyimide optical waveguide according to claim 1, wherein the first
index of refraction is larger than indices of refraction of the first
polyimide layer and the second polyimide layer.

7. The polyimide optical waveguide according to claim 6, layer and wherein
the indices of refraction of the first and second polyimide layers are
substantially identical.

8. The polyimide optical waveguide of claim 1 wherein the first polyimide
layer is rigid.

9. The polyimide optical waveguide of claim 1 wherein the first polyimide
layer is flexible.

10. An opto-electronic system comprising:

a first cladding layer;

a waveguide core material over the first cladding layer, wherein the
waveguide core material is optically conductive;

a second cladding layer contiguous to the waveguide core material and over
the first cladding layer, wherein the second cladding layer is
electrically conductive;

a third cladding layer contiguous to the waveguide core material and over
the first cladding layer, wherein the third cladding layer is electrically
conductive; and

a fourth cladding layer over the waveguide core material wherein the
waveguide core material and the first, second, third, and fourth cladding
layers are flexible.

11. The opto-electronic system according to claim 10, wherein the first and
fourth cladding layers consist essentially of a flexible polymer material.

12. The opto-electronic system according to claim 10, wherein the second
and third cladding layers are metallic.

13. The opto-electronic system according to claim 12, wherein the waveguide
core material, the first cladding layer, and the fourth cladding layer
electrically isolate the second and third cladding layers from each other.

14. The opto-electronic system according to claim 12, wherein the second
and third cladding layers each further comprise:

a plated metal layer; and

a metallic seed layer between the plated metal layer and the first cladding
layer.

15. The opto-electronic system according to claim 10, wherein the metallic
seed layer is between the first cladding layer and the waveguide core
material.

16. The opto-electronic system according to claim 10, wherein the first and
fourth cladding layers have a first and second index of refraction,
respectively, wherein the waveguide core material has a third index of
refraction at least 0.028 greater than the first and second indices of
refraction, and wherein the first and second indices of refraction are
substantially identical.

17. The opto-electronic system according to claim 10, wherein the waveguide
core material is an optically conductive adhesive.

18. A opto-electronic system comprising:

a first polyimide layer;

a second polyimide layer over the first polyimide layer;

a plurality of metal structures between the first polyimide layer and the
second polyimide layer, each of the plurality of metal structures
spatially separated from each other, each of the plurality of metal
structures comprising a plated metal layer over a first metallic seed
layer; and

an optically conductive adhesive between the first polyimide layer and the
second polyimide layer and between at least a portion of the plurality of
metal structures comprising a plated metal layer over a first metallic
seed layer; and

an optically conductive adhesive between the first polyimide layer and the
second polyimide layer and between at least a portion of the plurality of
metal structures, the optically conductive adhesive having a first index
of refraction.

19. The opto-electronic system according to claim 18, wherein the first
index of refraction is at least 0.028 greater than indices of refraction
of the first polyimide layer and the second polyimide layer.

20. The opto-electronic system according to claim 18, further comprising a
second metallic seed layer over the optically conductive adhesive, wherein
the first metallic seed layer is under the optically conductive adhesive.

Description

BACKGROUND OF THE INVENTION

This invention relates, in general, to an optical interconnect structure,
and more particularly, to an optical waveguide which is also capable of
conducting an electrical signal.

The speed at which data can be transferred from chip to chip, from board to
chip, from board to board, and even from computer to computer can be
limited by the speed of electron movement or transfer. One solution to
this speed limitation is to convert the electronic transfer process to a
photonic transfer process. Light emitting devices such as, for example,
vertical cavity surface emitting lasers (VCSELs) can convert electrical
signals to optical signals.

Light transmitting structures such as optical fibers are used to transmit
or conduct the optical signal from one location to another. However,
precise alignment is required for coupling the light transmitting
structures to the light emitting devices. Moreover, electrical contacts
carrying the original electrical signal must also be precisely aligned to
the light emitting devices. Consequently, two different alignment
procedures, which are difficult and expensive, must be successfully
completed prior to obtaining a functional electro-optical system.

Accordingly, a need exists for an optical waveguide which is both
manufacturable and inexpensive and which can also be used as an electrical
interconnect structure to facilitate alignment to a light emitting device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a partial perspective cross-sectional view of an optical
waveguide in accordance with the present invention; and

A light transmitting structure or optical waveguide of the present
invention can provide both optical and electrical conductivity. The
optical waveguide is capable of simultaneously transmitting multiple
optical and electrical signals. The optical waveguide has several cladding
layers which optically isolate several different regions of waveguide core
material through which the multiple optical signals are transmitted. To
provide a compact structure, portions of the cladding layers of the
optical waveguide are used as electrical conductors while the waveguide
core material is used as an electrical insulator for the electrical
conductors.

Turning to the figures for a more detailed description, FIG. 1 illustrates
a partial perspective cross-sectional view of an optical waveguide in
accordance with the present invention. Polyimide optical waveguide 34
includes a polyimide layer 10 which serves as a substrate on which
polyimide optical waveguide or optical waveguide 34 is built. Among other
materials, polyimide layer 10 is preferably comprised of flexible polymer
materials available under the trademark Kapton and manufactured by Dupont.
Upilex.TM. can also be used for polyimide layer 10. Polyimide layer 10 is
preferably about 25 to 200 microns thick. To facilitate the manufacturing
of optical waveguide 34, polyimide layer 10 is most preferably
Kapton.sup..TM. which is conventionally used for tape automated bonding
(TAB) tape. In fact, optical waveguide 34 can be manufactured using a
process similar to that used for the production of TAB tape. However,
polyimide layer 10 is not necessarily limited to the flexible polymer
materials listed above. For instance, in an alternative embodiment,
polyimide layer 10 can also be comprised of inflexible or rigid, materials
such resin filled epoxy, polyester materials, fiber reinforced materials,
or even semiconductors.

After formation of polyimide layer 10, metal structures 11, 12, 13, 14, 15,
and 16, and waveguide core material 17, 18, 19, 20, and 21, an additional
polyimide layer (not shown) is positioned over waveguide core material 17,
18, 19, 20, and 21 to serve as an upper cladding layer to keep the optical
signal within waveguide core material 17, 18, 19, 20, and 21. At a
minimum, optical waveguide 34 must have at least one waveguide core
material 17 which is located by, contiguous to, positioned adjacent to, or
between at least two metal structures 11 and 12.

Metal structures 11, 12, 13, 14, 15 and 16 can be fabricated using an
inexpensive and manufacturable process similar to that used for producing
metal leads or traces on TAB tape. For example, an adhesive can be used to
affix a metallic sheet to polyimide layer 10, and a chemical or laser
etching process can subsequently be used to define a desired configuration
of metal structures 11, 12, 13, 14, 15, and 16.

However, a preferred method uses a plating technique. First, a metallic
seed layer of gold or other appropriate seed material is sputtered over
polyimide layer 10. Second, a photoresist layer is patterned over the
metallic seed layer to define a desired configuration of metal structures
11, 12, 13, 14, 15, and 16. Third, the exposed portions of the metallic
seed layer are plated to a desired height preferably using an electrolytic
process to form plated metal layers 28, 29, 30, 31, 32, and 33. Next, the
photoresist layer is removed using known processes, and the subsequently
exposed portions of the metallic seed layer can then be etched away. The
above described preferred method produces an inert reflective surface
embodied as metal structures 11, 12, 13, 14, 15, and 16 which are
comprised of plated metal layers 28, 29, 30, 31, 32, and 33, respectively,
located over metallic seed layers 22, 23, 24, 25, 26, and 27,
respectively. The height of metal structures 11, 12, 13, 14, 15, and 16 is
dependent upon the heights of their respective metallic seed layers and
plated metal layers. For instance, the height of metal structure 11 is the
combined height of metallic seed layer 22 and plated metal layer 28, and
the height of metal structure 12 is the combined height of metallic seed
layer 23 and plated metal layer 29 and so on.

Waveguide core material 17, 18, 19, 20, and 21 is preferably an optically
transparent or optically conductive adhesive which holds optical waveguide
34 together. For instance, waveguide core material 17, 18, 19, 20, and 21
can be a material available under the trademark EPO-TEK 353ND from Epoxy
Technology, Inc. Waveguide core material or optically conductive adhesive
17, 18, 19, 20, and 21 can be dispensed or otherwise disposed onto
polyimide layer 10 and into regions between or contiguous to metal
structures 11, 12, 13, 14, 15, and 16 and then subsequently cured using
techniques such as air drying, UV light exposure, or heat treating
processes. In a preferred embodiment, the cross sectional area of a single
waveguide channel through which an optical signal is conducted is
approximately 45 to 250 microns.sup.2. The spacing between and the height
of metal structures 11, 12, 13, 14, 15, and 16 define the cross sectional
area of the waveguide channels.

Assuming polymide layer 10 is a flexible substrate, metal structures 11,
12, 13, 14, 15, and 16 and waveguide core material 17, 18, 19, 20, and 21
are, preferably, also flexible such that optical waveguide 34 can be bent
or positioned in a variety of configurations to facilitate its use and its
coupling to an active semiconductor device such as an optical device, an
opto-electronic device, or even a non-optical device. In this embodiment,
optical waveguide 34 can be easily coupled to the active device using
similar techniques as used for TAB tape.

Regardless of its composition, waveguide core material 17, 18, 19, 20, and
21 should have low optical transmission loss, low optical absorbtance, and
a controllable refractive index. Moreover, polyimide layer 10 should also
have a controllable refractive index as well. In particular, the index of
refraction of polyimide layer 10 should be less than the index of
refraction for waveguide core material 17, 18, 19, 20, and 21. In this
manner, the optical signal traveling through waveguide core material 17,
18,19, 20, or 21 will be contain by or will remain within the waveguide
core material. Preferably, the index of refraction for waveguide core
material 17, 18, 19, 20, and 21 is at least 0.028 greater or larger than
the index of refraction for polyimide layer 10.

Metal structures 11, 12, 13, 14, 15, and 16 also aid to keep the optical
signal traveling within waveguide core material 17, 18,19, 20, and 21.
However, instead of using a difference in the indices of refraction of two
materials, metal structures 11, 12, 13, 14, 15, and 16 use its reflective
properties to contain the optical signal within waveguide core material
17, 18, 19, 20, and 21. In other words, if the optical signal were to hit
a metal structure, the metal structure would reflect the signal back into
the waveguide core material.

While waveguide core material 17, 18, 19, 20, and 21 is optically
conducting, it is also preferably electrically insulating. Furthermore,
polyimide layer 10 is also preferably electrically insulating. As a
result, metal structure 11 is electrically isolated from metal structure
12 by polyimide layer 10 and waveguide core material 17. Similarly, metal
structure 12 is electrically isolated from metal structure 13 by polyimide
layer 10 and waveguide core material 18 and so on. With the above
described configuration, metal structures 11, 12, 13, 14, 15, and 16 are
all electrically isolated from each other and can be used as an electrical
interconnect structure.

Consequently, a more compact opto-electronic interconnect structure is
embodied as optical waveguide 34 because the optically conducting portions
also serve as the electrically insulating portions and the electrically
conducting portions are also used as the optically insulating portions.
Furthermore, if the electrical and optical conducting portions of optical
waveguide 34 are configured in a mirror image of electrical and optical
leads of an opto-electronic semiconductor device, only one alignment
procedure is necessary to provide both optical and electrical coupling to
the opto-electronic semiconductor device. Additionally, it is understood
that the waveguide, channels of optical waveguide 34 do not have to be
straight and parallel to each other as depicted in FIG. 1. Instead, the
waveguide channels can be curved or any other desired configuration.

Continuing with the next figure, FIG. 2 portrays a partial perspective
cross-sectional view of an alternative embodiment of an optical waveguide
in accordance with the present invention. Optical waveguide 47 has
waveguide core material 45 and 46 though which an optical signal is
conducted or transmitted, and optical waveguide 47 also has cladding
layers 40, 41, 42, 43, and 44 which keep or contain the optical signal
within the aforementioned waveguide core material. Cladding layers 40 and
41 can be similar in both function and composition to polyimide layer 10
of FIG. 1. Likewise, cladding layers 42, 43, and 44 of FIG. 2 can also be
similar in both function and composition to metal structures 11, 12, 13,
14, 15, and 16 of FIG. 1. Additionally, waveguide core material 45 and 46
of FIG. 2 can be similar to waveguide core material 17, 18, 19, 20, and 21
of FIG. 1. Therefore, cladding layer 43 is electrically isolated from
cladding layer 42 by waveguide core material 45 and cladding layers 40 and
41, and cladding layer 43 is also electrically isolated from cladding
layer 44 by waveguide core material 46 and cladding layers 40 and 41.

However, in an alternative embodiment, cladding layers 42, 43, and 44 are
strips or pieces of a similar material as cladding layers 40 and 41. In
this alternative embodiment, cladding layers 40, 41, 42, 43, and 44 are
all comprised of a flexible polymer material. Additionally, waveguide core
material 45 and 46 can be strips or pieces of a different flexible polymer
material which is optically conductive. Therefore, optical waveguide 47
can be several hundreds of meters long, can be dispensed from a spool in a
manner similar to that of thread or wire, and can be cut into appropriate
lengths. It is noted that regardless of their composition, waveguide core
material 45 and 46 and cladding layers 40, 41, 42, 43, and 44 of optical
waveguide 47 still need to comply with the index of refraction constraints
previously described for FIG. 1 where the index of refraction for the
waveguide core material is greater than the index of refraction for the
cladding layers. For this alternative embodiment, it is understood that
the indices of refraction for all of the surrounding cladding layers can
be substantially identical.

Another preferred or desired characteristic of the components of optical
waveguides 34 and 47 is high thermal stability. With high thermal
stability, the optical waveguides will be able to withstand the electronic
packaging and assembly processes used in semiconductor component
manufacturing. Procedures such as die attach, metallization, solder
reflow, and wire bonding are examples of high temperature processes to
which the optical waveguides may be subjected and from which the optical
waveguides should be able to survive.

Referring to the next figure, FIG. 3 portrays a partial perspective
cross-sectional view of yet another alternative embodiment of an optical
waveguide in accordance with the subject invention. Waveguide 61 of FIG. 3
is similar to waveguide 34 of FIG. 1. In particular, waveguide 61
includes: polyimide layers 50 and 51 which are similar to polyimide layer
10, seed layers or conductivity layers 52 and 60 which are similar to
metallic seed layers 22, 23, 24, 25, 26 and 27, plated metal layers 53,
54, 55, and 56 which are similar to plated metal layers 28, 29, 30, 31,
32, and waveguide core material 57, 58, and 59 which are similar to
waveguide core material 17, 18, 19, 20, and 21. The different strips of
waveguide core material 57, 58, and 59 are separated by at least a portion
of the metallic structures of FIG. 3.

Because the waveguide core material is surrounded by a metallic surface on
four sides, waveguide 61 of FIG. 3 uses the property of reflection to
contain an optical signal within the waveguide channels. Waveguide 61 does
not use a difference in the index of refraction of two materials to
contain an optical signal. Therefore, waveguide 61 is used mainly for
single mode transmission of light.

Therefore, in accordance with the present invention, it is apparent there
has been provided an improved optical waveguide which overcomes the
disadvantages of the prior art. The subject invention provides an optical
waveguide which can also be used to simultaneously conduct optical and
electronic signals. The present invention facilitates alignment to a
semiconductor device, is cost efficient, and is easily manufacturable.